1. Field of the Invention
The present invention relates to an electrical valve circuit connected to a variable voltage AC circuit system.
2. Description of the Prior Art
In a DC-AC rectifier for driving a synchronous motor or a DC-AC rectifier for imparting secondary excitation of a wound-rotor type induction motor, it is necessary to use an electrical valve circuit apparatus for the DC-AC rectifying which is connected to the variable voltage AC circuit system. In the electrical valve circuit apparatus employed for this a problem of the commutation between the electrical valves should be considered when the voltage of the AC circuit system is low. Heretofore, it has been proposed to employ a forcible commutating method by using commutating capacitors or an interrupting commutating method by using electrical valve circuit apparatus having a back-to-back connection.
However, in the former method, the commutating means for forcible commutation is rather complicated. Further, when high voltage application is desired, the voltage applied to the forcible commutating means and the electrical valves should be high resulting in rather high expense.
In the latter method, the current of the DC circuit system is interrupted whereby the power feed to the AC circuit system is also interrupted. Accordingly, the effective current is decreased and the torque of the motor is decreased. Also, the vibration and noise are generated by the change of the torque caused by the current interruption. Accordingly such large capacity apparatus has met with problems.